1. Field of the Invention
The present application relates to an encoder, and more particularly to an encoder operating in accordance with the transparent scanning principle.
2. Description of Related Art
In encoders that operate by the transparent scanning principle, a translucent partial disk, the so-called code disk, provides a measurement of angular displacement. On one side of the disk is an illuminating unit consisting of a light transmitter and a collimator lens, while on the opposite side there is a photoelectric scanning receiver. The light emitted by the light transmitter or from the illuminating unit is modulated by the angular displacement of the partial disk. The scanning receiver converts the modulated light signals into electrical measurement signals. The angular displacement can be incrementally or absolutely encoded. Encoders of this type are used in particular for measuring the angle and/or the angular velocity.
Encoders according to the prior art use a shaft during self-orientation which can also be designed as a hollow shaft and is connected nonrotationally with the drive shaft. The stator of the encoder is also connected nonrotatably by a torque support on the axle bearing housing of the drive shaft.
Encoders without their own bearings, so-called KIT encoders, that use the transparent scanning principle, have a code disk connected permanently with a drive shaft as well as a module fastened to the housing consisting of an illumination unit and a scanning receiver located on the opposite side of the code disk.
In order to keep the centrifugal forces acting on the partial disk low even at high rpm values and to prevent destruction of the partial disk consisting of glass or plastic, it is known from DE 40 13 936 A1 to place the partial disk in the end of the rotating shaft to be measured. The end of the shaft is located opposite a fixed housing end. An LED integral with the housing extends through a central opening in the partial disk. A reflector is placed in the coaxial bore at the end of the shaft and reflects the light from the LED as a parallel light bundle through the partial disk. The light passing through the partial disk is scanned by photodiodes located concentrically to the LED in the housing end. The arrangement of the LED and the scanning concentrically to the shaft permit a significant reduction in the diameter of the partial disk and hence a reduction of the centrifugal forces acting on the partial disk. The encoder can therefore be operated at much higher rpm values until centrifugal forces appear that may cause damage to the partial disk. Since the partial disk is inserted into the shaft, the encoder is engaged and supported at its outer circumference by the shaft. This support of the partial disk at its outer circumference as well as the smaller inertial moment of the disk permit higher rpm values.
In the foregoing encoder, the LED is a light source in the end of the housing. The photodiodes are placed around this LED in the end of the housing as well. When the encoder is assembled, the photodiodes must be adjusted individually in a costly manner in order to obtain a mutually concentric arrangement as well as an arrangement that is concentric to the axis of the shaft. Thus, it would be desirable to have an encoder having a simplified design and assembly that does not require, for example, individual adjustment of the photodiodes.
In accordance with principles of the invention, an encoder operates in accordance with the transparent scanning principle. The encoder includes a rotating shaft, a partial disk connected nonrotatably concentrically with the shaft, an illuminating unit, and a scanning receiver located axially in front of the end of the shaft. The illuminating unit is connected nonrotatably centrally with the shaft and rotates with the shaft on the side of a code disk that faces away from the scanning receiver. The scanning receiver includes a monolithic photoreceiver array which is formed with photosensitive sensor areas associated with an angular measurement of the partial disk.
In the encoder according to the invention, the illuminating unit is inserted coaxially into the shaft or connected nonrotatably with it and turns with the shaft. The scanning receiver contains a monolithic photoreceiver array and is located centrally and axially in front of the end of the shaft. The scanning receiver may be a SoC OptoChip (SoC: System-on-Chip), and may include semiconductor components in addition to the photoreceiver array, for example components related to signal processing. In some embodiments, each track of the partial disk has four groups of photoreceivers as photosensitive areas of the monolithic array which are each offset 90xc2x0. The signals from each of the groups of photoreceivers may be averaged to compensate for possible eccentric motion of the partial disk. Thus, requirements for the bearing and running tolerances of the partial disk may therefore be reduced. By designing the scanning receiver as a monolithic array, the need for individual adjustment of the photoreceiver on assembly is greatly reduced, if not eliminated. Since the phototransmitter is not located concentrically within the photoreceiver, it is possible to arrange the photoreceivers with a smaller diameter so that the radial dimensions can be reduced further, thus increasing the admissible rpm values.
The encoder according to the invention may be designed with or without its own bearing. In the embodiment in which the encoder includes its own bearing, the drive shaft and the encoder shaft are connected nonrotatably with one another and the encoder shaft contains the illuminating unit as well as the code disk. The stator of the encoder and the axle bearing housing of the drive shaft are connected together nonrotatably by a torque support.
In a KIT version without its own bearing, the code disk as well as the illumination unit are connected nonrotatably with the drive shaft to be measured. Thus, in one embodiment, the phototransmitter is enclosed in a bore made coaxially in the end of the shaft and turns with the drive shaft. The partial disk bearing the angle code may be located between the illuminating unit and the scanning receiver. The scanning receiver may be fastened to the housing. In contrast to the previous encoders that lack their own bearings, only the scanning receiver is attached nonrotatably to the housing.